Method and Apparatus for Distributing a Media Content Service

ABSTRACT

A media content service is distributed across a distribution network. A plurality of media content service representations ( 80 A,  80 B) for the media content service are generated. Each of the media content representations ( 80 A,  80 B) has a different bit rate. Each of the media content service representations ( 80 A,  80 B) is divided into segments ( 81 A,  81 B). Segments ( 81 A,  81 B) of the plurality of media content service representations are aligned in time. The segments are transmitted as a sequence of bursts ( 82 A,  82 B). The bursts of the plurality of media content service representations are aligned in time. There is a window period ( 83 ) between successive bursts during which media content is not transmitted. Each of the media content service representations is transmitted as a multicast. Each of the generated media content service representations ( 80 A,  80 B) has a respective bit rate and each of the segments ( 81 A,  81 B) is transmitted at a higher bit rate to create the window periods ( 83 ) between successive bursts ( 82 A,  82 B).

TECHNICAL FIELD

This invention relates to a method and apparatus for distributing amedia content service and to a method and apparatus for receiving amedia content service.

BACKGROUND

Traditional telecommunications systems have evolved rapidly in recentyears to encompass considerably more capability than simply delivering atelephony service. Internet technology based on data packets andassociated transmission protocols, for example the Internet Protocol(IP), has now superseded the old space switching techniques anddedicated transmission resources during a telephone call or datatransmission session.

The convergence of flexible delivery technologies has enabled servicesto be mixed in the same physical resource so that for example, intelephony networks, subscribers are now able to receive audio-visualcontent, including relayed broadcast television and radio services, aswell as access internet resources, all via their telephone lines.Similarly, the expansion of wireless networks have enabled a user toreceive a range of communications services over a wireless deliverychannel.

A Digital Subscriber Loop (xDSL) modem/router, which is widespread inhomes, acts as a common assembly point for all domestic communicationstraffic, such as telephony, computer-based web browsing, email, homeshopping or streaming of video material. As will be appreciated by askilled person, the capacity of the digital subscriber loop has to beshared among the services being provided to the subscriber at anyparticular time.

There is considerable interest in receiving “streamed” services whichdeliver a stream of video and/or audio material to users. The contentcan be live content or it can be recorded content which is requested ondemand. In an Internet Protocol Television (IPTV) system, video andaudio content is delivered to users via Internet Protocol (IP) deliverymechanisms. These types of services require a continuous anduninterrupted flow of data. A premises may have multiple terminals whichare capable of receiving media content. The demands placed on thedelivery channel can vary according to how many terminals requirecontent at a particular time.

Streamed media content services can be provided by one of two methods:Unicast or Multicast. Unicast provides the subscribing receiver with adirect and unique two-way path through the delivery network all the wayback to the media server supplying the required data stream. The mainstreaming activity is managed on a one-to-one basis between the receiverand the source server. The network between the source server andreceiver typically comprises a series of intermediate servers installedat network nodes which are not directly involved in the service but onlysupport the transfer of a packet stream. Typically the protocols used tosupport the transmissions are simple forms of Internet Protocol (IP)itself augmented by one or more higher layer protocols to provide flowcontrol. These protocols extend across the span of the link between thesource server and a given receiver. This method of distribution iswasteful of network capacity because, at busy times and for popularmedia streams, many copies exist in the network simultaneously and thiscontributes to network congestion.

A Unicast system can support Adaptive Bit Rate Streaming (ABR). Thisallows some form of rate adaptation. A given service is encoded at aselection of different bit rates (known as representations), withsynchronised boundary points at defined locations (e.g. every 50frames). For each representation, content between successive boundarypoints is converted into a discrete file. Clients fetch a segment of oneof the representations in turn. If a higher or a lower bit rate isrequired, the next segment is fetched from one of the otherrepresentations. The segments are constructed such that there is nodiscontinuity in decoded pictures/audio if the client switches betweenrepresentations at the boundary points. This system requires the unicasttwo-way path between source and receiver to request files and deliverthe requested files.

Multicast makes more efficient use of bandwidth by sharing contentstreams among several receivers. Intermediate network routers are nowmore closely involved in the service delivery such that some control andmanagement functions are delegated from the source server. This controlis supported by more extensive protocols devised for this type ofapplication such as Protocol Independent Multicast (PIM) and InternetGroup Multicast Protocol (IGMP). When a new receiver requests a givenmedia item, the network router system finds an existing stream of thatcontent already in the network and directs a copy of it to that newreceiver from an appropriately near network node. A new receiver has tobe able to join this existing stream under controlled conditions that donot adversely affect existing receivers. Any receiver in this group alsohas to be able leave the stream, or pause its consumption, withoutaffecting the others. There is now a clear separation of control andmanagement from the content. This complicates the control functionbecause several receivers will have different instantaneous bit rate andcontrol needs and so the segmentation of the stream as it passes througha given network has to deal with the conflicting demands. These aremanaged directly between the receiving device itself and the nearestupstream router/server that will mediate control functions including anyneed to refer back to earlier network servers or even the originalsource server.

Currently, each TV service has to be set up with a conservativeconfiguration. This can be a capped Variable Bit Rate (VBR) with a safecap value. This is because it is not possible to change bit rate of aservice on the fly without interrupting the TV service.

In summary, while unicast systems can support an adaptive bit rate of astreamed service over time, this comes at a significant cost in terms ofresources of the delivery network to support a large number of unicastpaths, and may require additional servers located near to clients of thenetwork to maintain acceptable response times. Multicast systems canmore efficiently use bandwidth resources of the network, but do notsupport adaptive bit rate needs of client devices.

The present invention seeks to provide an alternative way ofdistributing media content services, such as video services.

SUMMARY

An aspect of the present invention provides a method of distributing amedia content service across a distribution network comprising receivingor generating a plurality of media content service representations forthe media content service. Each of the media content representationshave a different bit rate. The method further comprises dividing each ofthe media content service representations into segments. The segments ofthe plurality of media content service representations are aligned intime. The method further comprises transmitting the segments of each ofthe plurality of media content service representations as a sequence ofbursts. The bursts of the plurality of media content servicerepresentations are aligned in time. There is a window period betweensuccessive bursts during which media content is not transmitted. Each ofthe media content service representations is transmitted as a multicast.Each of the received or generated media content service representationshas a respective bit rate. The transmitting comprises transmitting eachof the segments at a higher bit rate to create the window periodsbetween successive bursts.

Advantageously, the dividing comprises inserting data which indicates aboundary of a segment and the step of transmitting each of the segmentsuses the inserted data to determine a boundary of a segment.

Advantageously, the method further comprises transmitting an informationelement which indicates at least one of: an end of a burst; an advancenotice of an end of a burst. The information element can be carriedwithin each of the bursts.

Advantageously, each of the media content service representations istransmitted as a first multicast with a multicast destination IPaddress, and the method comprises transmitting a second multicast whichcarries the information element, wherein the second multicast has thesame destination IP address as the first multicast.

Advantageously, the method further comprises adapting the media contentdata into packets for transport over the distribution network, and thestep of transmitting each of the segments as a burst operates on thepackets.

Advantageously, the method further comprises transmitting informationabout which media content service representations are available forselection.

The method can be performed by a node of the distribution network suchas a head end node, or by any other node or combination of nodes.

Another aspect of the invention provides a method of receiving a mediacontent service across a delivery channel. The media content service isavailable in a plurality of representations each having a different bitrate. Each of the plurality of media content representations aretransmitted as a sequence of bursts, with the bursts aligned in time.There is a window period between successive bursts during which mediacontent is not transmitted. The method comprises receiving a first ofthe media content service representations of the media content serviceacross the delivery channel as part of a multicast of the first mediacontent service representation. The method further comprises selecting asecond of the media content service representations of the media contentservice for delivery across the delivery channel. The method furthercomprises sending a request to leave the multicast of the first mediacontent service representation. The method further comprises sending arequest to join a multicast of the second media content servicerepresentation. The method further comprises receiving the second mediacontent service representation as part of the multicast of the selectedmedia content service representation. The leaving and joining of therespective multicasts occurs during one of the window periods.

Advantageously, the method further comprises receiving an informationelement which indicates at least one of: an end of a burst; an advancenotice of an end of a burst and using the information element todetermine when the sending a request to leave the multicast of the firstmedia content service representation and the sending a request to join amulticast of the second media content service representation can occur.

Advantageously, the method further comprises receiving information aboutwhich media content service representations are available for selection.

Advantageously, the step of sending a request to join a multicast occursbefore the step of sending a request to leave a multicast.

Advantageously, the method is for receiving a composite media contentservices stream, comprising one or more media content services, themethod further comprising dynamically determining the required mediacontent services for delivery across the delivery channel. The methodfurther comprises dynamically determining the bandwidth available fordelivery of the composite media content services stream. The methodfurther comprises selecting one of the plurality of media contentservice representations of a required media content service for deliveryacross the delivery channel depending on at least the bandwidthavailable for the composite media content services stream.

Advantageously, the selecting depends on at least one of: a type ofdevice associated with the media content service and a priority of themedia content service.

Advantageously, a higher priority information element is allocated toreal-time media content services and a lower priority informationelement is allocated to recorded media content services.

Advantageously, a value of a priority information element associatedwith the media content service depends on at least one of: acategorisation of the media content service; a device which is toconsume the media content service.

Advantageously, the method further comprises storing data received inbursts of the received media content service and sending informationabout the availability of the stored data to adaptive bit rate devicesserved by the apparatus.

Advantageously, the method is performed at a subscriber apparatus.

Advantageously, each burst comprises a plurality of Internet Protocolpackets.

Advantageously, the media content service is a video service.

Advantageously, at least two of the plurality of media content servicerepresentations for the video service have a different video resolution.

Another aspect of the invention provides apparatus for distributing amedia content service across a distribution network. The apparatuscomprises an interface for receiving a media content service. Theapparatus comprises segmenting apparatus arranged to generate aplurality of segmented media content service representations for themedia content service, each of the media content representations havinga different bit rate and divided into segments, wherein the segments ofthe plurality of media content service representations are aligned intime. The apparatus comprises an output unit arranged to transmit thesegments of each of the plurality of media content servicerepresentations as a sequence of bursts, wherein the bursts of theplurality of media content service representations are aligned in time,and wherein there is a window period between successive bursts duringwhich media content is not transmitted. The output unit is arranged totransmit each of the media content service representations as amulticast. Each of the received or generated media content servicerepresentations has a respective bit rate and the output unit isarranged to transmit each of the segments at a higher bit rate to createthe window periods between successive bursts.

Another aspect of the invention provides apparatus for receiving a mediacontent service across a delivery channel. The media content service isavailable in a plurality of representations each having a different bitrate. Each of the plurality of media content representations aretransmitted as a sequence of bursts, with the bursts aligned in time,and wherein there is a window period between successive bursts duringwhich media content is not transmitted. The apparatus comprises acommunication interface for receiving a first of the media contentservice representations of the media content service across the deliverychannel as part of a multicast of the first media content servicerepresentation. The apparatus comprises a controller coupled to thecommunication interface which is arranged to select a second of themedia content service representations of the media content service fordelivery across the delivery channel. The controller is further arrangedto send a request to leave the multicast of the first media contentservice representation. The controller is further arranged to send arequest to join a multicast of the second media content servicerepresentation. The controller is further arranged to receive the secondmedia content service representation as part of the multicast of theselected media content service representation, wherein the leaving andjoining of the respective multicasts occurs during one of the windowperiods.

Embodiments of the present invention provide a way of delivering mediacontent services (e.g. video services) in a manner which makes efficientuse of resources of the distribution network, while allowing a clientdevice to dynamically alter the bit rate of the media content service.The client device may need to adapt the bit rate due to varyingbandwidth demands required of the delivery channel, such as a change tothe number of media content services used at any particular time, or achange to the bandwidth of the delivery channel (e.g. with a wirelessdelivery channel).

Embodiments of the present invention provide a way of allowing a clientdevice to dynamically alter the bit rate of the media content servicewithout significant disruption to the media content service.

The functionality described here can be implemented in hardware,software executed by a processing apparatus, or by a combination ofhardware and software. The processing apparatus can comprise a computer,a processor, a state machine, a logic array or any other suitableprocessing apparatus. The processing apparatus can be a general-purposeprocessor which executes software to cause the general-purpose processorto perform the required tasks, or the processing apparatus can bededicated to perform the required functions. Another aspect of theinvention provides machine-readable instructions (software) which, whenexecuted by a processor, perform any of the described methods. Themachine-readable instructions may be stored on an electronic memorydevice, hard disk, optical disk or other machine-readable storagemedium. The machine-readable medium can be a non-transitory medium. Themachine-readable instructions can be downloaded to the storage mediumvia a network connection.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 shows a typical system for delivering services to subscriberpremises over a delivery channel;

FIG. 2 shows a modified system corresponding with FIG. 1 in accordancewith an embodiment of the invention;

FIG. 3 shows processing performed on a media content stream to create aset of segmented representations in accordance with an embodiment of theinvention;

FIG. 4 shows processing performed on representations of a media contentstream of different bit rates to form bursts in accordance with anembodiment of the invention;

FIG. 5 shows a method of processing media content which can be performedat one or more nodes of a distribution network;

FIGS. 6 and 7 show time lines of an operation to change the bit rate ofa media content stream received at a subscriber apparatus;

FIG. 8 shows apparatus in accordance with an embodiment of theinvention;

FIG. 9 shows part of the apparatus of FIG. 8 in more detail;

FIG. 10 shows packets used to carry media content data;

FIG. 11 shows a media content distribution network in more detail;

FIG. 12 shows transmission of multicasts to a subscriber apparatus;

FIG. 13 shows a method of receiving media content data which can beperformed at a subscriber apparatus;

FIG. 14 shows a method of receiving media content data which can beperformed at a subscriber apparatus;

FIG. 15 shows apparatus in accordance with an embodiment of theinvention;

FIGS. 16 and 17 show utilisation of a delivery channel over a period oftime in accordance with an embodiment of the invention

FIG. 18 shows processing apparatus for a computer-based implementation.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a typical system for providingcommunication services to subscriber premises 5. The various resourcesshown contribute to the delivery of a group of services to a subscriberover the digital subscriber loop 32 between a premises 5 and thenetwork.

At the subscriber side shown on the right-hand side of FIG. 1, a numberof different devices are present that may require services to beprovided over a digital subscriber loop. Typically, such devices mayinclude: a telephone 2; computers 4, 6; video decoders 8 and 10;together with other terminals denoted generically by terminal 12. Thesedevices 2-12 are coupled to a digital subscriber loop (xDSL) router 40over a local subscriber network 16, which is typically provided by anEthernet or a wireless network.

On the network side shown on the left-hand side of FIG. 1, a digitalsubscriber loop (xDSL) access multiplexer (DSLAM) 20 is coupled via adistribution network 60 to a plurality of service sources 24-30. Thedistribution network 60 can be an Internet Protocol (IP) baseddistribution network in which all of the services 24-30 are transportedusing IP. The term xDSL includes Asymmetric Digital Subscriber Line(ADSL) and variants (ADSL2, ADSL2+), Very high bit-rate DigitalSubscriber Line (VDSL) and other variants.

The services 24-30 shown at the left are examples of various types ofservice, all of which may be combined using Internet Protocol (IP) fordelivery across the digital subscriber loop to the consumer, where theyare separated again for the individual destination devices in the home,as will be understood by a skilled person. The Internet Protocol (IP)infrastructure at the Service Provider end is configured differently ondemand for each subscriber, as will be understood by a skilled person,and additional DSLAMs 21, 22 are shown for completeness.

A video source 30 is shown supplying an input to the distributionnetwork 60. There are various types of video service. A first type ofvideo service is live video that is delivered in real time. A secondtype of video service uses video data which is stored as video files inservers. These video files may be watched in real time as if they werelive video or may be transferred in non-real time for later playback atthe subscriber's premises. Typically, this video service type might beused for a film library that enables a film to be downloaded by asubscriber for immediate viewing or stored for later use. The storedvideo files may be for video service recording content accessed inaccordance with the requirements of an individual subscriber, forexample in accordance with a digital video recorder (DVR) schedule of asubscriber. In some embodiments this non real-time video service contentmay be deferred by being temporarily buffered in the network when thedigital subscriber loop is being heavily utilised, and being moved tothe local storage when capacity across the digital subscriber loopbecomes available.

FIG. 2 is a schematic diagram corresponding with FIG. 1 and showingadditional elements in accordance with embodiments of the invention.Elements in FIG. 2 having the same or similar function to correspondingelements in FIG. 1 have been given the same reference numerals.

In FIG. 2, apparatus 41 provided at a subscriber end 5 includes acontrol function which is able to select a representation of a mediacontent stream, at a particular bit rate, and can join a multicast whichprovides that representation. The control function is able todynamically select between representations over a period of time, andcan leave a multicast of one representation and can join a multicast ofanother representation. In some embodiments, apparatus 42 provided at asubscriber end 5 can arbitrate between multiple demands of devices 2-12.Apparatus 42 can conveniently form part of xDSL router 40 or it can be aseparate device which is connected to the xDSL router 40. Alternatively,the functionality of apparatus 42 can form part of a device 2-12connected to the xDSL router 40. Policy, which can be used by theapparatus 42 to arbitrate between demands, can be located 44 at thesubscriber apparatus and/or in the network 62. Distribution network 60includes additional processing functions 69 to form multicast streamswhich provide a set of representations of a service at different bitrates. As described below, these representations are transmitted as asequence of bursts with window periods between successive bursts whichallow a subscriber apparatus to join and/or leave a multicast of arequired representation. The processing functions 69 can be performed bya distribution head end 61 or by any other node or combination of nodes.

In embodiments of the invention, each media content service is madeavailable in a plurality of media content service representations, eachhaving a different bit rate. For example, a video service 1 can be madeavailable in three video service representations 1 a, 1 b, 1 c and videoservice 2 can be made available in three video service representations 2a, 2 b, 2 c. In different embodiments, and for different video services,the number of video services representations available for selection maybe greater or less than 3, as decided by a skilled person. One or moreother properties of the video service can vary among therepresentations, such as video resolution, frame rate. Audio rates (ornumber of audio channels) can also vary between representations. Forexample, consider representations 1 a, 1 b, 1 c have progressivelyhigher bit rates. At least one of the higher bit rate representations(e.g. 1 c) can have a higher resolution than one of the lower bit raterepresentations (e.g. 1 a, 1 b).

FIG. 3 shows processing performed on a media content stream 85 to form aset of representations 80A, 80B at different bit rates. In this exampleembodiment the media content is video. Each video service 85 is coded asa set of representations 80A, 80B. For simplicity, FIG. 3 shows ConstantBit Rate (CBR) rate streams but the coding does not have to be constantbit rate. For example, it could be variable bit rate (VBR) or variablebit rate with a particular peak or mean bit rate value, such as cappedVariable Bit Rate (VBR). The values of 5 Mbit/s and 7 Mbit/s are examplevalues, and it will be appreciated that a representation can begenerated at any other desired value and that any number ofrepresentations may be generated. Each representation is divided into asequence of segments 81A, 81B. Segment boundaries 82 are aligned, suchthat the segment boundaries of one representation 80A are aligned intime with the segment boundaries of another representation 80B. Acorresponding segment in each of the representations correspond to thesame frames of video content. As will be explained, this allows areceiver to switch between representations at one of the segmentboundaries 88 and perform a smooth transition between representations.Advantageously, a segment boundary 88 occurs at the end of a Group ofPictures (GOP) 83.

A feature of compressed video data stream syntax is the Group ofPictures (GOP) which comprises a sequence of complete and contiguousvideo pictures or frames. The Group of Pictures (GOP) begins with aframe selected to be a master frame, denoted as an “I” or Intra Framefor that sequence. This I Frame is coded without reference to any otherframe in the sequence and exploits only spatial redundancy within the Iframe. Since I frames may be decoded without reference to pictureinformation in any other frame, I frames can be relied upon as astarting point for a decoder.

The other frames or pictures in a Group of Pictures (GOP) can be codedvery efficiently by exploiting temporal redundancy in the imagesequence. These frames are coded so that information is transmitteddescribing differences between the current frame and already codedreference frames temporally adjacent to it. Such frames are of twotypes: one type is a Predicted or P frame type, which is predicted andcoded only from one direction of the image sequence, for example fromearlier frames in the image sequence. The other type is a Bidirectionalframe or B frame type, which are predicted from both forward andbackward directions of the sequence, for example by interpolation fromearlier and later frames in the video sequence.

As will be appreciated, the success of compression algorithms inachieving low bit rates is because the P and B Frames use smalleramounts of data to encode the picture than an I Frame and are morenumerous in a Group of Pictures (GOP). An I Frame uses more data toencode the picture and so these are relatively sparse in a Group ofPictures (GOP). The I frame is very useful when streams are interruptedor suffer occasional errors because it resets the decoder with anindependent Frame.

Thus a Group of Pictures (GOP) can be structured as a sequence of I, Pand B Frames in an arbitrary pattern e.g. IBBPBBP . . . until the next IFrame is inserted. The length of the Group of Pictures (GOP) will affectcoding efficiency because it uses predominantly small coded frame sizes.Such a Group of Pictures (GOP) is known as a Closed Group of Pictures(GOP) because it is self-contained and has defined entry points at the IFrame for decoders such that coded pictures following the I frame do notreference any pictures before the I frame. This is useful for thisapplication because the I Frames allow points in a stream at which a bitrate change can be made without necessarily disrupting the decoder.

Advantageously, the video service representations of a video servicehave aligned segment boundaries 88 so that their GOP patterns arealigned. Data for the different bit rate representations 80A, 80B ispacketised. FIG. 3 shows a sequence of Transport Stream (TS) packets84A, 84B for each representation. The number of packets per segmentvaries according to the bit rate of the representation. The higher bitrate stream 80B has more packets than the lower bit rate stream 80A.

Advantageously, segment boundaries are marked by introducing BoundaryPoints (BP) that mark the location of the segment boundaries in thesegmented streams 80A, 80B where it is possible to switch cleanlybetween the representations and deliver the segments using multicast.These BPs can also act as reference points in the stream for any otherpurpose. The BP can be inserted in a continuous stream, for example as ashort gap, or the segmented media data stream can be momentarily pausedso that the absence of data allows time for receivers to adjustthemselves to any changing circumstances including new receivers joiningthe stream or existing ones pausing or leaving it.

Conveniently, media data can be encoded according to Motion PicturesExperts Group MPEG-2 format and encapsulated into MPEG TransportStreams, but it will be appreciated that segments 81A, 81B can also becoded and encapsulated in any other suitable format, such as the MPEG-4File Format.

FIG. 4 shows processing performed on segmented media content streams80A, 80B, to form a set of bursts at different bit rates. As will beexplained below, the segmented streams 80A, 80B are fed into a multicastplayout block that makes each segment play out in less time than theinterval between the boundary points. The purpose of this is todeliberately introduce a window period between transmitted bursts 82A,82B. The duration of the window period 83 is sufficient for a multicastreceiver to leave one representation of the stream and join a differentrepresentation at a different bit rate. The switching between multicastscan be achieved using multicast Leave and Join messages. The duration ofthe window period 83 is sufficient for a receiver to send requests tojoin and leave respective multicasts and for switches and routers in thedistribution network 60 to action the requests. The multicasts bursts82A, 82B have a higher bit rate compared to the respective segmentedrepresentations 80A, 80B. The bit rate of the bursts is increased by afactor of a, where:

$a = \frac{{segment}\mspace{14mu} {duration}}{\left( {{{segment}\mspace{14mu} {duration}} - {{window}\mspace{14mu} {duration}}} \right)}$

For clarity, FIG. 4 shows the start point of the bursts 82A, 82B alignedin time with the start point of the segments 81A, 81B of representations80A, 80B, to illustrate the comparison in length of the segments 81A,81B compared to the bursts 82A, 82B. It will be appreciated that a shortbuffer period (of at least the length of the window period) is requiredbefore the burst can be played out, which would be represented in FIG. 4as an offset along the time axis between the start of segments 81A, 81Band the start of bursts 82A, 82B.

The start of bursts of different content services can be aligned in time(i.e. all bursts of all services start at the same time) or the starttimes can be staggered with respect to one another. Aligning the starttimes of bursts of different services in time can have an advantage ofallowing a quicker channel change between services.

One consequence of adding window periods 83 to the transmitted streamsis that the flow of content becomes jittered beyond the level thatnormal packet transmission produces. This can be accommodated bybuffering provided within the reception apparatus. The window periodduration will influence the amount of jitter produced and so there is anoptimum size that is related to the worst case response times of thechain of routers/servers delivering the content. This time is taken forswitches to recognise and implement all those steps that are required tokeep the stream flowing including the potential need for the multicastcontent to be found at or near the source server. For segment durationsof the order of about 2 seconds, a possible value of the window periodis around 330 milliseconds. The window size is a function of theresponsiveness of the network to support multicast leave and joinfunctions and it will be appreciated that the window period can bemodified to a higher or lower value. More generally, the window periodcould have a value selected from the range of 0.1 ms and 10 seconds and,more advantageously, the range 10 ms-350 ms. As switches and routersincrease in performance, it is possible that the window duration can bereduced to the lower en d of the ranges stated above.

Internet Group Management Protocol (IGMP)v3 provides an explicit LEAVEmessage, allowing multicast clients to signal they no longer require themulticast to be forwarded to them. This can be implemented in IPswitches as “Fast Leave”, where the multicast forwarding is stoppedimmediately when the LEAVE message is received. There is also an IGMPJoin message to join a multicast.

FIG. 5 shows a method according to an embodiment of the invention. Themethod can be performed by a node in the distribution network 60. Step101 comprises generating a plurality of media content servicerepresentations for the media content service, each of the media contentrepresentations having a different bit rate. Step 102 comprises dividingeach of the media content service representations into segments.Segments of the plurality of media content service representations arealigned in time. The dividing can comprise marking boundary pointsbetween segments by inserting signalling data into the data stream foreach representation. Step 103 comprises transmitting the segments ofeach of the plurality of media content service representations as asequence of bursts. So, the segments of a first representation aretransmitted as a first sequence of bursts, the segments of a secondrepresentation are transmitted as a second sequence of bursts, and soon. Bursts of the plurality of media content service representations arealigned in time and there is a window period between adjacent burstsduring which media content is not transmitted. So, the bursts of thefirst representation are time-aligned with the bursts (representing thesame part of the content) of the second representation, and with thebursts of any other representations, where the time-aligned burstsrepresent the same part of the content (e.g. the same video frame). Eachof the media content service representations is transmitted as amulticast. Each of the generated media content service representations80A, 80B has a respective bit rate and the transmitting comprisestransmitting each of the segments 81A, 81B at a higher bit rate tocreate the window periods 83 between successive bursts 82A, 82B.

FIGS. 6 and 7 show time lines of an operation to change the bit rate ofa media content stream received at a subscriber apparatus. FIG. 6 showstwo multicast representations 87A, 87B of different bit rates which aretransmitted across distribution network 60. A subscriber apparatus canselect one of these representations by joining the multicast. Thesubscriber apparatus receives bursts 82B of representation 87B (high bitrate) during periods t1-t2 and t3-t4. At time t4 the subscriberapparatus sends signalling to leave the multicast of representation 87B(high bit rate) and to join the multicast of representation 87A (low bitrate). At time t6 the subscriber apparatus receives a burst 82A ofrepresentation 87A (low bit rate). The switch between leaving multicastof representation 87B and joining the representation of multicast 87Aoccurs during the window period t4-t6. Thus, the subscriber apparatusreceives a seamless flow of data for the media content service, eventhough a change in the delivery bit rate has occurred. The order ofsignalling can be reversed, such that the request to join the newmulticast 87A is sent before sending the request to leave the existingmulticast 87B. A worst case time for joining a multicast is longer thana worst case time for leaving a multicast, and so it is advantageous toissue the command to join first.

FIG. 7 shows further detail of the process to change the bit rate of amedia content stream received at a subscriber apparatus. There arevarious ways in which a subscriber apparatus can detect the end of aburst and the start of the period when the apparatus can safely switchbetween representations. It is possible to configure a terminal tomeasure a “time out” period at the beginning of a window period, butthis introduces an undesirable latency. It is more advantageous to sendsignalling which explicitly indicates an end of a burst, or which givesadvance notice of when an end of burst will occur. This signals to thesubscriber apparatus when it is possible to safely switch betweenrepresentations, with minimal delay. Advantageously the locations in thedata stream where the switching between streams can safely occur areexplicitly signalled within, or alongside, the multicasts, thus allowinga receive device to immediately be aware that a switching opportunityexists. Signalling can be provided, for example, in IP packets thatshare the same multicast destination address as the IP packets carryingthe media content service, but different UDP destination port numbers,allowing simple detection and discrimination at the receive device. Thishas the advantage that whenever the multicast that carries the mediacontent is routed (in response to IGMP joins) the signalling would alsoautomatically follow. Alternatively, the signalling could be deliveredin a separate multicast having a separate multicast destination address,with the subscriber apparatus joining/leaving both the multicastcarrying the media content and the multicast carrying the end of burstsignalling. Another possibility is to include a trailer indicating anend of burst in the media multicast. Examples include: adding trailerdata at the end of the last UDP datagram, sending an empty datagram, orchanging a parameter such as the sync byte (normally hexadecimal 47) toa different value. Other possibilities are to send an informationelement which gives advance notice of an end of a segment, such as anumber of TS packets remaining in the current segment, an RTP sequencenumber of the last IP packet in the current segment, or some othersuitable advance notice.

The scheme described above can be used when representations at differentbit rates to minimise disruption to the delivery and presentation ofmedia content. When a terminal first joins a service the joining couldoccur during the window or it may occur at any time.

FIG. 8 shows apparatus for creating a set of segmented representationsand forming bursts in accordance with an embodiment of the invention.The apparatus 69 shown in FIG. 8 can be provided at a distribution headend node 61. The apparatus receives an input feed 85 of content (e.g.video) per service. A set of encoders/transcoders 65 generate, inparallel, a set of representations of the content at a set of differentbit rates. The representations can differ in another quantity, such asvideo resolution. A timing generator 64 outputs a signal that determinesthe boundary point of the segments. For example, this can output asignal once per 50 frames (2 seconds), or at any other suitable timeinterval. The signal output by generator 64 is applied to all of theparallel encoders 65 for that service. Advantageously, the set of coders65 can close the GOP (see FIG. 3) and a unit 76 can insert in-bandsignalling such as a Random Access Point (RAP) and a Boundary Point(BP). The outputs of the set of coders 65 are the set of representations80A, 80B which have time-aligned segments. The representations 80A, 80Bare applied to a burst generator unit 67 which creates the bursts (82A,82B, FIG. 4) separated by window periods (83, FIG. 4). Unit 67 plays outdata at a higher bit rate than the bit rate at which data was receivedto create the bursts. Unit 67 recognises the start and end of segments81A, 81B in the representations 80A, 80B by detecting signalling (e.g.the in-band BP signalling inserted by the encoders 65).

GOPs do not necessarily have to be closed at the boundary point. Bydefault they are closed. However closed GOPs are slightly moreinefficient than open GOPs, so it is possible that open GOPs could alsobe used. The use of open GOPs has a disadvantage that, when changingrepresentations, there may be some visible impairment to the picturesfor the first GOP of the new representation. If representation switchesoccur infrequently, then open GOPs may be acceptable.

Advantageously, multicast address information is added at the IPadaptation stage, which can occur at the head end 61, or at a nodedownstream of the head end. Multicast IP datagrams have a destination IPaddress is set to be in a range reserved for multicast.

The apparatus shown in FIG. 8 can operate on data at one of variouspossible levels. In one advantageous scheme, data is encoded intoTransport Stream (TS) packets at a normal rate and the burst generatorunit 67 operates on TS packets. Transport Stream packets can be adaptedinto IP packets 66 before burst generator unit 67, or after unit 67. Analternative is to form bursts before any packetisation (at TS or IPlevel), but this is less desirable.

FIG. 8 shows several possible places where end of burst signalling canbe added. Each encoder 65 can include an end of burst signal insertionunit 77 which adds an information element indicating the end of a burstinto the encoded data, which includes a header of the TS packets.Alternatively, the burst generator unit 67 can include an end of burstsignal insertion unit 78 which is arranged to insert an informationelement indicating the end of a burst into each of the bursts. Where endof burst signalling is provided as a multicast (with same or differentaddress), the multicast can be generated at apparatus 61.

Typically, a network operator will receive a media content service feedfrom a service provider and will then convert the feed to the rightcharacteristics for the distribution network and end users of thenetwork. FIG. 8 shows a set of encoders/transcoders 65 which operateupon an input data stream 85 to form the multiple representations atdifferent bit rates. However, for situations where a set ofrepresentations at different bit rates already exist, it is possible forthe input 85 to be a set of representations at different bit rates. Themultiple representations can be generated at a point of origin into thedistribution network 60 or could be supplied by a service provider.

The multiple representations of a video service can include StandardDefinition (SD) and High Definition (HD) representations of the samecontent. The distribution network may receive an SD feed and an HD feedof the content as inputs 85 or just one (SD or HD) feed with transcoding65 to generate the other of the SD and HD representations. So, input 85may be a high quality HD feed and encoders 65 transcode the HD feed togenerate lower bit rate, lower resolution, versions of the content.

FIG. 9 shows further detail of the playout unit 67 of the apparatus ofFIG. 8. The burst generator unit 67 comprises a parallel set of units70. Each unit 70 has an input 71 for receiving a segmented stream 80A,80B of one of the representations. Data is applied to a buffer 72. Thebuffer operates according to a First In First Out (FIFO) principle. Datais also applied to a detector 73 which detects signalling (e.g. BPsignalling) within the data stream indicating the start and end of asegment. Detector 73 applies an output to a unit 74 which controlsplayout of data from the buffer 72. A segment of data is played out inone continuous block, which can be in the form of a sequence of TS or IPpackets carrying TS packets. Unit 74 controls the playout rate, which isgreater than the rate at which data was originally received. Data isoutput 75 in burst form, as explained above.

FIG. 10 shows an example format of packets used to carry media contentdata. Video data comprises data representing GOPs. A GOP comprises oneor more of I frames, B frames and P frames. GOPs are carried inTransport Stream (TS) packets. Other possible formats include MPEGDynamic Adaptive Streaming over HTTP (MPEG-DASH). A TS packet has alength of 188 bytes, of which 4 (or more) bytes are header bytes. Aheader of a TS packet can include signalling which indicates a BoundaryPoint. The BP can be signalled within the adaptation header (AH) of a TSpacket. For an IP-based distribution network, TS packets are carriedwithin the payloads of IP packets. Typically, 7 TS packets areaggregated into one IP packet. Typically, a segment will comprisemultiple IP packets. IP packets may be spaced by a short inter-packetgap, which is considerably shorter than the window period. Segments maystart with one or more TS packets before the first TS packet carrying(video) content data. One example of this is where encryption is used.The TS packet immediately before the first packet carrying video contentcontains data needed for provisioning the correct decryption key.

FIG. 11 shows distribution networks 60 that may be used to multicastmedia content data, and protocols used to carry the data and controldata flows. The upper part of FIG. 11 represents a distribution networkfor IPTV services using xDSL to subscriber premises. A distribution headend 91 receives inputs 30 from content (e.g. video) sources, which canbe live or recorded sources. The distribution network between the headend 91 and a DSL access module comprises distribution nodes 92, whichinclude IP routers. The subscriber loop 32 to a subscriber premises isterminated, at the subscriber side, by a customer premises router, whichcan also be called a Residential Gateway (RG). Data is carried over thedistribution network using IP. The Internet Group Management Protocol(IGMP) is a communications protocol used on the IP distribution networkto establish multicast group memberships. Protocols such as IGMP areused to support the communication between upstream routers/servers andsupport the Leave and Join actions described above. Protocol-IndependentMulticast (PIM) is another protocol used within the distributionnetwork. Within the customer premises, content can be distributed bywired network protocols (e.g. Ethernet) or wireless network protocols(e.g. IEEE 802.x). PIM is used between routers, whereas IGMP is layer 2and is used by switches. The protocols register a destination's interestin receiving a multicast, either for itself, or to be forwarded to oneor more next destinations. When there is a leave, the traffic stops ifthere is nothing else at that destination that is joined. The fast leaveoperation terminates that flow immediately, whereas without fast leave,a switch will respond to a leave command sending out a request to see ifany destination is still interested, keeping the traffic flowing until atimeout occurs. The fast leave operation is used in embodiments of theinvention to switch between multicast representations.

The lower part of FIG. 11 represents a wireless distribution network,and comprises a Mobile Network Access Portal 93, Distribution NetworkNodes 94 and wireless Base Stations 95. There is a wireless deliverychannel between a base station 95 and a receiving device 96, which canuse a 3G or 4G wireless protocol. As before, the IGMP can be used toestablish multicast group memberships, and Protocol-IndependentMulticast (PIM) can be used within the distribution network. The mobilereceiving device 96 can share content with other devices using a wiredor wireless connection. In the wireless system, base station 95 canperform the join multicast/leave multicast functionality (equivalent tothat performed at the customer premises router 40) to share the limitedresource of the overall wireless network capacity of the cell betweenmultiple mobile receiving devices 96. Alternatively, a mobile receivingdevice 96 can perform the join multicast/leave multicast functionality,to share the limited resource of the wireless connection between device96 and the base station 95 between multiple services required by thedevice 96.

FIG. 12 shows transmission of multicasts to a subscriber apparatus 41.Nodes within the distribution network can comprise a headend, a networknode (such as a Central Office, and a multi-cast capable DSLAM. The(S,G) notation denotes S=Source Address, G=Multicast DestinationAddress. G1 a, G1 b are multicasts carrying different representations ofone particular media content service. The multicasts are shown beingsent in parallel from the headend 91 to node 92, because the multicastsare received by subscriber apparatus 41 at multiple premises within thenetwork, but the multicasts are sent one at a time from the node 92 tothe subscriber apparatus (RG) 41 based on the RG first requesting one ofthe representations (S, G1 a) and then changing to the other of therepresentations (S, G1 b) for a bit rate adjustment on that service. TheRG 41 can concatenate the incoming multicasts, which may switch betweenrepresentations, and form a single continuous multicast output to aconventional receiver 4, 6, 8, 10, 12. As explained above, thefunctionality of switching between multicast streams may be performed bya device 4, 6, 8, 10, 12 connected downstream of the RG, and that devicewould perform the concatenation.

FIG. 13 shows a method of receiving media content data which can beperformed at a subscriber apparatus. Step 111 comprises receiving afirst of a plurality of the media content service representations of themedia content service across the delivery channel as part of a multicastof the first media content service representation. Step 112 comprisesselecting a second of the media content service representations of themedia content service for delivery across the delivery channel. Steps113 and 114 can be performed in any order. Step 113 comprises sendingsignalling to leave the multicast of the first media content servicerepresentation. Step 114 comprises sending signalling to join amulticast of the second media content service representation. Step 115comprises receiving the second media content service representation aspart of the multicast of the selected media content servicerepresentation. The window period between successive bursts has aduration which is sufficient to allow a terminal to join the multicast,and the leaving and joining of the respective multicasts occurs duringthe window period.

Operating in the manner described above has an advantage in that onlyconventional networking equipment is needed in the core network of theoperator's distribution network (downstream of the modified head end).Another advantage is that the network capacity is more efficiently usedbecause when more than one subscriber is watching the same content (atthe same rate) then the multicasts can be used by both, thereby reducingthe core network bandwidth needed.

Another aspect of subscriber apparatus is that it can perform anarbitration function that repeatedly assesses the requested servicesinto the home. This includes media content (e.g. video) services andother data services. The arbitration can select between a plurality ofdifferent bit rate representations of the same media content service andthereby determine an optimal selection of the services based on criteriasuch as overall capacity of the delivery channel, service category, userpreferences, operator policies, decode/display types and servicecurrent/next quality metrics. One quality metric is complexity ofencoding content and the arbiter can be arranged to allocate a greaterportion of the shared bandwidth of the delivery channel when the contentis more complex and a higher data rate is required to encode thecontent. As demands dynamically change over a period of time, it ispossible to select a higher or lower bit rate representation of eachrequested service to fulfil all of the requests.

The mechanism described above makes it possible to select betweendifferent bit rate representations of a content media service in aseamless manner. Each subscriber apparatus can dynamically select adifferent mix of service rates depending on local needs of the devicesserved by that subscriber apparatus.

FIG. 14 shows a method of determining media content services which canbe performed at the subscriber apparatus. Step 121 comprises dynamicallydetermining the required media content services for delivery across thedelivery channel. Step 122 comprises dynamically determining thebandwidth available for delivery of the composite media content servicesstream. Step 123 comprises selecting one of the plurality of videoservice representations of a required media content service for deliveryacross the digital subscriber loop depending on at least the bandwidthavailable for the composite media content services stream.

The selecting at step 123 can depend on at least one of: a type ofdevice associated with the media content service and a priority of themedia content service. A higher priority information element isallocated to real-time media content services and a lower priorityinformation element is allocated to recorded media content services. Apriority information element associated with the media content servicecan depend on a categorisation of the media content service or a devicewhich is to consume the content.

FIG. 15 shows subscriber apparatus in accordance with an embodiment ofthe invention. An xDSL router 40 connects to a subscriber loop 32, whichforms a two-way delivery channel to/from the premises. A controller 42performs the method shown in FIG. 13 for selecting betweenrepresentations of a media content service, and sending signalling toleave and join respective multicasts which deliver thoserepresentations. Controller 42 comprises a representation selection unit46. Representation selection unit 46 receives inputs from a bandwidthmonitor 45 and a burst end detector unit 47. Bandwidth monitor 45monitors bandwidth of the delivery channel 32. Burst end detection unit47 can detect in-band signalling, or out-of-band signalling, whichindicates the end of a burst and a start of the window period duringwhich a change of the representation can be made. A memory 50 isaccessible by the representation selection unit 46. Memory 50 storesdata which can include data 55 about each the requested services. Data55 can include an indication 56 of the device which is to consume thecontent and an indication 57 of the priority of the requested service.The priority can be determined by using the stored policy 44. Memory 50stores data 51 about the currently selected representation per mediacontent service. Memory 50 stores data 52 about availablerepresentations per service (e.g. data indicating that representationsare available for service X at bit rates of 2, 5, and 7 Mbit/s). Memory50 can store policy data 44 which is used by the representationselection unit 46 to select between representations. As an example, therepresentation selection unit 46 may arbitrate between demands of: (i) achildren's TV programme which is to be viewed on a small screen displaydevice in a child's bedroom; (ii) a major sports event which is to beviewed on the household's main, large screen, display device. Therepresentation selection unit 46 may also arbitrate between demands ofother communications services, such as internet traffic. The policy thatinforms the choice can be partially defined centrally (e.g. sportsprogrammes should be prioritised over kids programmes) and partiallylocally (e.g. definitions of the relative priorities of display devicesat the premises, such as the main display having a higher priority thanthe display in the child's bedroom.)

Subscriber apparatus 41 can store data 53 which is received for at leastone of received media content services, and can send information aboutthe availability of the stored data to devices served by the apparatus41. The marking of segments of content has already been performed in thedistribution network, such as by the head end. Adaptive Bit Rate (ABR)clients retrieve a sequence of files corresponding to segments ofcontent. The data of a received burst that is received according to anembodiment of the invention, corresponds to a segment that can beconsumed by an Adaptive Bit Rate client. Subscriber apparatus 41 canmake the segment available for retrieval by a client by a file fetchmechanism rather than the push mechanism used in the multicast delivery.Support for this type of operation is provided by an ABR support unit 58which can form part of the subscriber apparatus 41 or some devicedownstream of the subscriber apparatus, such as a set-top box (STB). ABRsupport unit 58 converts each segment of received content data into anABR segment that can be retrieved by an ABR device. Unit 58 then acts asa proxy and serves those files to the ABR client. In this scenario, onlyone rate would ever be visible to the ABR client, which is the rate fromthe multicast that the representation selection unit 46 has selected toreceive. Unit 58 can support conversion into HTTP Live Streaming (HLS)or Dynamic Adaptive Streaming over HTTP (DASH) formats.

FIG. 16 shows bandwidth utilisation of a delivery channel, such as asubscriber loop 32 to a premises, over a period of time. Two videoservices are shown. Over the period of time shown, video service 1starts at a high bit rate and then switches to a lower bit rate. Videoservice 2 starts at a low bit rate and then switches to a higher bitrate. At any point in time, the overall bandwidth consumed is the sum ofvideo service 1+video service 2. Any spare capacity is available forhigh-speed data or other communication services.

FIG. 17 shows a more detailed example of allocation of loop capacityover a period of time in response to changes in services demanded in anexemplary illustration. The horizontal axis in FIG. 17 shows time, whilethe vertical axis in FIG. 17 indicates video service representations andtheir bit rates.

The session begins at point 170 with only Voice over Internet Protocol(VoIP) and High Speed Internet Access services in place. The Voice overInternet Protocol (VoIP) and High Speed Internet Access services arepermanently available for use on demand and are allocated fixedcapacity. At this time there are no video services present.

At point 172 a first video service, Service A, is requested by a memberof the subscriber's household and consists, in this example, of a highdefinition television (HDTV) version of that service initially set to bedelivered at 8 Mbps (Mbit per second). It is to be displayed in realtime on the main household large screen display device and may be, forexample, a live news channel.

This progresses for a short time and then at point 174 another member ofthe household, possibly located elsewhere, selects Service B which maybe, for example, an educational channel and this is provided to acomputer terminal at standard definition and initially delivered at 2Mbps. These services do not cause capacity conflicts and so are able tobe delivered simultaneously. The appropriate segment capacity level fromthose available so that the service qualities requested can bedelivered.

At point 176, some time later, the two existing services are joined by athird service, Service C, requested by yet another household member andwhich may be for example in the exemplary embodiment a sports channelrequested in high definition (HD). The sum of the service requirementsof the three services A, B and C now exceeds the subscriber loopcapacity, and so some adaptation is necessary according to theinvention.

In the exemplary embodiment this adaptation results in the selection ofvideo services representations for the high definition (HD) services Aand C having a bit rate of 5 Mbps. The bit rate of the video servicerepresentation selected for Service B remains unchanged at 2 Mbps.

This situation remains in place for some time further until at point 178a pre-set recording function is initiated by the domestic Digital VideoRecorder (DVR) device. The content required is service D in highdefinition (HD), which is to be delivered in real time at 5 Mbps. Thisset of capacities is within the loop capacity, and as a result the highdefinition (HD) video services representation is selected.

Shortly after this at point 180 another pre-set recording request forservice E is initiated and this demand requires a change in the bitrates of the existing services. This is managed using the resources ofthe invention so that services A, B and C being live TV services do notchange bit rates and the new service is accommodated by selecting newvideo services representations for the DVR services so as to reduce thebit rates for the two to 2.5 Mbps which means, if the quality criteriaare retained, that the content will be delivered at half speed takingtwice as long to complete. This is a reasonable adjustment that balancesthe live and DVR service requirements.

This combination of services continues for a while until at point 182service A is terminated making 5 Mbps available. The system chooses toleave real time services unchanged and to allocate this capacity to theDVR channels restoring real time speed delivery at 5 Mbps each. Thus avideo services representation with increased bit rate is selected.

It is possible for quality criteria and capacities to be such that anumber of DVR events, which may be set up by different household membersso that no one is fully aware of the DVR schedule, cannot be satisfiedas well as the current real time services being consumed. This mayrequire the system to intervene and issue on screen messages to anappropriate channels warning of the likelihood of the loss of a digitalvideo recording (DVR) event unless some quality criteria are changed sothat for example the standard definition (SD) video servicesrepresentation is selected instead of the High Definition video servicesrepresentation or a live service is terminated.

Later still, at point 184, service B terminates making available afurther 2 Mbps. This additionally available bandwidth is used to enhanceservice C by selecting a video services representation for service Chaving a higher bit rate.

Embodiments of the invention continue is this fashion seamlesslyadjusting capacity allocations so that demand is managed intelligentlyand always within the loop capacity and always maintaining servicequality criteria.

FIG. 18 shows an exemplary processing apparatus 200 which may beimplemented as any form of a computing and/or electronic device, and inwhich embodiments of the system and methods described above may beimplemented. Processing apparatus 200 can be provided at one of thedistribution network nodes, or at a subscriber apparatus 41. Processingapparatus may implement the method shown in any of FIGS. 5, 13 and 14.Processing apparatus 200 comprises one or more processors 201 which maybe microprocessors, controllers or any other suitable type of processorsfor executing instructions to control the operation of the device. Theprocessor 201 is connected to other components of the device via one ormore buses 206. Processor-executable instructions 203 may be providedusing any computer-readable media, such as memory 202. Theprocessor-executable instructions 203 can comprise instructions forimplementing the functionality of the described methods. The memory 202is of any suitable type such as read-only memory (ROM), random accessmemory (RAM), a storage device of any type such as a magnetic or opticalstorage device. Additional memory 204 can be provided to store data 205used by the processor 201. The processing apparatus 200 comprises one ormore network interfaces 208 for interfacing with other network entities.

Modifications and other embodiments of the disclosed invention will cometo mind to one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed and that modifications andother embodiments are intended to be included within the scope of thisdisclosure. Although specific terms may be employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation.

1. A method of distributing a media content service across adistribution network comprising: receiving or generating a plurality ofmedia content service representations for the media content service,each of the media content representations having a different bit rate;dividing each of the media content service representations intosegments, wherein the segments of the plurality of media content servicerepresentations are aligned in time; and transmitting the segments ofeach of the plurality of media content service representations as asequence of bursts, wherein the bursts of the plurality of media contentservice representations are aligned in time, and wherein there is awindow period between successive bursts during which media content isnot transmitted, wherein each of the media content servicerepresentations is transmitted as a multicast, and wherein each of thereceived or generated media content service representations has arespective bit rate and the transmitting comprises transmitting each ofthe segments at a higher bit rate to create the window periods betweensuccessive bursts.
 2. A method according to claim 1 wherein the dividingcomprises inserting data which indicates a boundary of a segment and thestep of transmitting each of the segments uses the inserted data todetermine a boundary of a segment.
 3. A method according to claim 2further comprising transmitting an information element which indicatesat least one of: an end of a burst; an advance notice of an end of aburst.
 4. A method according to claim 3 wherein the information elementis carried within each of the bursts.
 5. A method according to claim 3wherein each of the media content service representations is transmittedas a first multicast with a multicast destination IP address, and themethod comprises transmitting a second multicast which carries theinformation element, wherein the second multicast has the samedestination IP address as the first multicast.
 6. A method according toclaim 1 further comprising adapting the media content data into packetsfor transport over the distribution network, and the step oftransmitting each of the segments as a burst operates on the packets. 7.A method according to claim 1 wherein the generating comprises receivinga stream of media content data and transcoding the stream to form theplurality of representations of the media content data at the pluralityof different bit rates.
 8. A method according to claim 1 furthercomprising transmitting information about which media content servicerepresentations are available for selection.
 9. A method of receiving amedia content service across a delivery channel, the media contentservice being available in a plurality of representations each having adifferent bit rate, wherein each of the plurality of media contentrepresentations are transmitted as a sequence of bursts, with the burstsaligned in time, and wherein there is a window period between successivebursts during which media content is not transmitted, the methodcomprising: receiving a first of the media content servicerepresentations of the media content service across the delivery channelas part of a multicast of the first media content servicerepresentation; selecting a second of the media content servicerepresentations of the media content service for delivery across thedelivery channel; sending a request to leave the multicast of the firstmedia content service representation; sending a request to join amulticast of the second media content service representation; and,receiving the second media content service representation as part of themulticast of the selected media content service representation, whereinthe leaving and joining of the respective multicasts occurs during oneof the window periods.
 10. A method according to claim 9 furthercomprising receiving an information element which indicates at least oneof: an end of a burst; an advance notice of an end of a burst and usingthe information element to determine when the sending a request to leavethe multicast of the first media content service representation and thesending a request to join a multicast of the second media contentservice representation can occur.
 11. A method according to claim 9further comprising receiving information about which media contentservice representations are available for selection.
 12. A methodaccording to claim 9 wherein the step of sending a request to join amulticast occurs before the step of sending a request to leave amulticast.
 13. A method according to claim 9 for receiving a compositemedia content services stream, comprising one or more media contentservices, the method further comprising: dynamically determining therequired media content services for delivery across the deliverychannel; dynamically determining the bandwidth available for delivery ofthe composite media content services stream; and selecting one of theplurality of media content service representations of a required mediacontent service for delivery across the delivery channel depending on atleast the bandwidth available for the composite media content servicesstream.
 14. A method according to claim 13 wherein the selecting dependson at least one of: a type of device associated with the media contentservice and a priority of the media content service.
 15. A methodaccording to claim 14 wherein a higher priority information element isallocated to real-time media content services and a lower priorityinformation element is allocated to recorded media content services. 16.A method according to claim 14 wherein a value of a priority informationelement associated with the media content service depends on at leastone of: a categorisation of the media content service; a device which isto consume the media content service.
 17. A method according to claim 9further comprising: storing data received in bursts of the receivedmedia content service; sending information about the availability of thestored data to adaptive bit rate devices served by the apparatus.
 18. Amethod according to claim 9 performed at a subscriber apparatus.
 19. Amethod according to claim 9 wherein each burst comprises a plurality ofInternet Protocol packets.
 20. A method according to any claim 9 whereinthe media content service is a video service.
 21. A method according toclaim 20 wherein at least two of the plurality of media content servicerepresentations for the video service have a different video resolution.22. Apparatus for distributing a media content service across adistribution network comprising: an interface for receiving a mediacontent service; segmenting apparatus arranged to generate a pluralityof segmented media content service representations for the media contentservice, each of the media content representations having a differentbit rate and divided into segments, wherein the segments of theplurality of media content service representations are aligned in time;an output unit arranged to transmit the segments of each of theplurality of media content service representations as a sequence ofbursts, wherein the bursts of the plurality of media content servicerepresentations are aligned in time, and wherein there is a windowperiod between successive bursts during which media content is nottransmitted, wherein each of the media content service representationsis transmitted as a multicast, and wherein each of the received orgenerated media content service representations has a respective bitrate and the output unit is arranged to transmit each of the segments ata higher bit rate to create the window periods between successivebursts.
 23. Apparatus for receiving a media content service across adelivery channel, the media content service being available in aplurality of representations each having a different bit rate, whereineach of the plurality of media content representations are transmittedas a sequence of bursts, with the bursts aligned in time, and whereinthere is a window period between successive bursts during which mediacontent is not transmitted, the apparatus comprising: a communicationinterface for receiving a first of the media content servicerepresentations of the media content service across the delivery channelas part of a multicast of the first media content servicerepresentation; a controller coupled to the communication interfacewhich is arranged to: select a second of the media content servicerepresentations of the media content service for delivery across thedelivery channel; send a request to leave the multicast of the firstmedia content service representation; send a request to join a multicastof the second media content service representation; and, receive thesecond media content service representation as part of the multicast ofthe selected media content service representation, wherein the leavingand joining of the respective multicasts occurs during one of the windowperiods.
 24. A computer program product comprising a machine-readablemedium carrying instructions which, when executed by a processor, causethe processor to perform the method of claim 1.